EU law is clear in my mind that the Airline must rebook you into available seats in the same cabin, not the same booking class which is an arbitrary commercial invention by Airlines to control pricing.

Dr. Böse agrees and had a court injunction opened against Lufthansa for precisely this purpose. He won and the court threatened LH with fines of €250,000 for not complying with the law. Info Here.

Lufthansa are appealing this, obviously to gain time until their rescue package is completed, but IMHO will almost certainly lose. The mere fact that one needs to get a court order to force LH to comply with both their own conditions of carriage and EU law is frankly a condemnation of institutions empowered to enforce these laws.